Turf mowers comprise one or more cutting units, either reel or rotary cutting units or cutting units comprising individual cutting blades, carried on a frame for cutting grass over a wide cutting swath. In a gang mower configuration, a plurality of cutting units are usually arranged in front and rear rows with the cutting units in each row being laterally spaced from one another if there is more than one cutting unit in each row. The cutting units in the respective rows are laterally staggered relative to the cutting units in the other rows so that the gaps between the cutting units in one row are covered by the cutting unit(s) in another row. This allows the gang configuration of cutting units to cut an unbroken swath of grass during a single pass of the gang mower. Typically, gang mowers of this type comprise riding mowers that are operated by an operator carried on a seat provided on the mower.
Turf mowers of this type traditionally have been powered by an internal combustion engine carried on the frame of the mower, typically either a diesel engine or a gasoline powered engine. In many cases, the engine powers one or more hydraulic pumps that supply pressurized hydraulic fluid to a plurality of hydraulic motors or hydraulic actuators. At least one hydraulic motor powers one or more of the ground engaging wheels of the mower to self-propel the mower over the ground. Other hydraulic motors power the cutting units. Various hydraulic actuators are used in a lift and lower system to raise and lower various lift arms that support the cutting units for movement between a lowered grass cutting position in which the cutting units engage the turf and a raised transport position in which the cutting units are elevated above the turf. Various other hydraulic actuators may be used in a power steering system. The Reelmaster® Series 5010 mowers manufactured and sold by The Toro Company are well known gang mowers of this general type in which the cutting units comprise reel cutting units.
Today, environmental regulations continue to tighten and energy prices continue to rise. For example, the EPA has promulgated a set of stringent emission regulations governing non-road diesel engines known as the Tier IV regulations. Relatively small diesel engines having less than 25 horsepower (hp) are currently able to meet the Tier IV regulations without being unduly expensive. Unfortunately, however, diesel engines having more than 25 hp, which include many diesel engines used in gang mowers, will not meet the Tier IV regulations without the addition of relatively expensive emission reduction equipment. This additional cost will have to be either passed on to customers or absorbed by the manufacturer, neither of which is desirable. It would be an advantage to devise a system that would allow gang mowers that have traditionally been powered by diesel engines over 25 hp to be powered by a diesel engine below 25 hp without the customer feeling or noticing any difference in the performance of the mower or the power characteristics of the mower.